CRYSTAL STRUCTURES AND MAGNETIC PROPERTIES OF Bi0.84La0.16Fe0.98Ti0.02O3 POLYCRYSTALLINE CERAMIC
Chu Thi Anh Xuan, Pham Truong Tho, Nguyen Van Dang* University of Sciences - TNU
ABSTRACT
Sample Bi0.84La0.16Fe0.98Ti0.02O3 was prepared by the solid-state reaction method. The analysis of XRD pattern indicated the coexistence of multi-phase crystal structure of the R3c rhombohedral and the Imma orthorhombic. The Rietveld refinement method was used to calculate the lattice parameter, bonding angle Fe–O–Fe, and bonding length Fe–O. Sample showed a typical of weak ferromagnetic. The presence of multi-phase crystal structure affected strongly the magnetic properties of sample. Especially, the vertical hysteresis shift (exchange bias) was observed in sample. We believed that the origin of this effect come from the magnetic interaction at phase boundary of two crystal structures.
Keywords: BiFeO3, Crystal structures, Magnetic properties; multiferroics materials; multiphase crystal
INTRODUCTION*
Recently, multiferroic BiFeO3 (BFO) has attracted much attention because of the coexistence of both ferroelectric and magnetic properties at room temperature, making BFO become a potential candidate for many applications in magnetoelectric devices. Bulk BFO materials have a rhombohedrallly distorted perovskite structure (space group R3c) with a high ferroelectric Currie temperature (TC 1100 K) and antiferromagnetic (AFM) Néel temperature (TN 643 K) [1]. The magnetic order in BFO is G-type AFM with a cycloid-type spatial spin modulation. The cycloidal spin spirals with periodicity of ~62 nm risen from the combination of exchange and spin-orbit interactions produces spin canting away from perfect AFM ordering [2]. In addition, due to the lack of inversion symmetry in the R3c structure, BFO shows weak ferromagnetic (wFM) order associated with Dzyaloshinky- Moriya (DM) interactions [3]. A disadvantage of BFO is the existence of a high leakage current due to impurity phases (such as Bi2Fe4O9, Bi25FeO40) and oxygen vacancies generated during the synthesis process. The improvement of this disadvantage can be
*Tel: 0983 009975; Email: dangnv@tnus.edu.vn
based on the partial substitution of Bi by a rare-earth or alkaline-earth element.
For the case of La-doped BFO, the structural change and distortion depend on La concentration that could significantly suppress impurity phases, reduce oxygen vacancies. This breaks spiral spin structure, and lower leakage current. Additionally, FM order would emerge in the AFM phase. It has also been found that the crystal structure of La-doped BFO is strongly dependent on synthesis and processing conditions. Using sol-gel method to prepare Bi1-yLayFeO3, one found cubic and orthorhombic structures in the samples with y = 0.2 – 0.4 and x = 0.5, respectively [4]. The tetragonal structure was reported on Bi0.8La0.2FeO3 prepared by solid state reaction and sol-gel methods [5,6].
Particularly, a phase separation was found in Bi1-yLayFeO3 as y = 0.16 [7], resulting in unusual magnetic and piezoelectric properties. By substituting Fe by a transition metal, such as Mn or Ti, one can improve remarkably the magnetic property of BFO.
This is understood as an increase of supper- exchange interactions between Fe ions.
Doping Ti into the Fe site could break the cycloidal spin order and reduce oxygen vacancies, because Ti is formed in an oxidation state Ti4+ higher than Fe3+.
However, no detailed study on the influence of the crystal structure on magnetic properties of (La, Ti) co-doped BFO materials, particularly around the rhombohedral- orthorhombic phase transfor-mation. Dealing with this issue, we prepared polycrystalline Bi0.84La0.16Fe0.98Ti0.02O3 samp-les, and studied their crystal structures and magnetic properties by using an X-ray diffractometer, and a magnetometer. The results obtained from above measurements indicated the presence of phase separation between rhombohedral-orthorhombic crystal structures and a weak ferromagnetic due to the structural transformation and doping effect.
EXPERIMENTAL DETAILS
Polycrystalline sample of Bi0.84La0.16- Fe0.98Ti0.02O3 (BLFTO) was prepared by solid-state reaction, using high-purity oxides of Bi2O3, La2O3, Fe2O3 and TiO2 as precursors. These powders in stoichiometric masses were thoroughly mixed by using an agate mortar and pestle, and then pressed into disc-shaped pellets. After several times of calcining, the pellet was finally annealed at 900 oC for 20 h. The crystal structure was studied by using an X-ray diffractometer (Miniflex Rigaku) equipped with a Cu-K
radiation ( = 1.5405Å). Magnetization measurements were performed on a vibrating sample magneto-meter (VSM). All the investigations were carried out at room temperature.
RESULTS AND DISCUSSION
The Rietveld refinement of XRD data was performed using GSAS-II program. A multi- phase models of R3c rhombohedral and another crystallographic symmetry such as Pbam orthorhombic, Imma orthorhombic, or I4/mcm tetragonal were carried out. Base on the weighted profile residual (Rwp) and good- ness of fit (G.O.F), the best fit was obtained with the R3c + Imma models as shown in Figure 1. The structural parameters obtained with the help of the refinement are listed in
Table 1. The R3c phase has lattice parameters of a = b = 5.573 Å, c = 13.812 Å, which is in agreement with the JCPDS card No. 86-1518.
Fig. 1. Rietveld refined XRD pattern using two phases model of R3c and Imma.
Table.1. Refined structural parameter Space group R3c
(52%)
Imma (48%)
a (Å) 5.573 5.633
b (Å) 5.573 7.821
c (Å) 13.812 5.640
Cell volume (Å3) 371.51 248.47 Fe-O-Fe (degree) 149.3 156.8
<dFe-O>(Å) 2.053 2.020
<dBi-O>(Å) 2.445 2.441
Rwp (%) 4.4
1.06 G.O.F
The Imma phase has lattice parameters of a = 5.633 Å, b = 7.821 Å, and c = 5.640 Å. The value of Fe – O bond length in octahedron is approximately equal to 2 Å. And, the Fe – O – Fe differs from 1800 of the ideal perovskite structure, which is implication the distorted R3c symmetry. The partially distorted crystal structure may affect the magnetic properties due to the suppression of cycloidal spin structure. To further confirmed Rietveld results, we plotted the simulated patterns of R3c and Imma phase with experimental XRD pattern, as seen in Fig. 2. It is clear that two phases model of R3c and Imma phases were fitted well with the experimental pattern. Our results are consisted with previous reported on Bi1-yLayFeO3 that the Imma occurred at y = 0.25 [8]. The presence of small among of Ti
Intensity (arb. units)
(~2 wt%) in Bi0.84La0.16FeO3 can stabilize Imma phase. The collinear spin structure of Imma orthorhombic have been studied by neutron diffraction on Bi0.8La0.2Fe0.8Mn0.2O3
compound [9]. The presence of multiphase crystal structure was showed a novel effect such as the double hysteresis loop, unusual piezoelectric properties, and the exchange bias effect. Therefore, the coexistence of R3c and Imma phases with different spin structure, and magnetic anisotropy may lead interesting magnetic properties.
20 30 40 50 60 70
31.5 32.0 32.5
simulated
Si Si
measured
intensity (arb.units)
2 (degree)
Imma R3c measured
simulated Si
Fig. 2. Experimental XRD pattern and the simulated XRD pattern of R3c and Imma phases.
Inset shows the representation of R3c rhombohedral and Imma orthorhombic structures Moreover, Figure 3 shows the hysteresis loop of BLFTO sample measured at 300 K with two cycles hysteresis loop. The hysteresis loops are showed unsaturated magnetization at 10 kOe with small remanent magnetization (Mr) confirmed wFM behavior of sample. The weak ferromagnetic behavior of BLFTO sample has origin from the partial suppression of cycloidal spin structure due to the structural transformation from R3c to Imma phase. The opening hysteresis loop with large Mr value compared with pure BFO confirms the improvement of magnetic properties by co-doping La and Ti to BFO. It is interesting to observe the vertical hysteresis shift in BLFTO sample. The vertical hysteresis shift is widely known as the minor loop effect, which only present after cycling hysteresis
loop at high applied field, or cooling sample in external field from high to low temperature [10]. Therefore, the vertical hysteresis shift observed in BLFTO is unexpected and cannot come from the minor loop effect.
-10 -8 -6 -4 -2 0 2 4 6 8 10
-0.10 -0.05 0.00 0.05 0.10 0.15
M (emu/g)
H (kOe) 1st
2nd
Fig. 3. The M(H) loops of BLFTO sample measured with two cycles (1st and 2nd) the minor loop is characteristic by the vertical shift and unclosed loop. But, as seen in 2nd loop, the unclosed hysteresis loop can be solved in second loop cycle. In BFO-based compounds, the previous studies also observed the vertical shift, but the origin of this effect is remain unclear [11]. The inhomogeneous magnetic phase from two crystal structures could be the origin of this effect. In second loop, the remanent magnetization and coercivity reduce strongly comparison with the first loop. The critical reduction was only observed for second loop, whereas high order loop cycles their values are almost stable (Figure 4). The hysteresis loop in Figure 4 was measured after pre- applied -10 kOe on sample. It is clear that the direction of hysteresis shift is depended on the sign of applied field. From these results confirm that the spin pinning is the origin of the vertical hysteresis shift. The spin pinning effect possibly originates from the magnetic interaction at phase boundary of two crystal
structure. Depending on the sign of applied field, the spin pining governs the hysteresis shift behavior. Performing hysteresis loop at high applied field may destroy the pinned spin to reveal a high symmetry of hysteresis loop, as normally observed in previous reports [12].
-10 -5 0 5 10
-0.10 -0.05 0.00 0.05 0.10
-1 0 1 2 3
-0.01 0.00
M (emu/g) 0.01
H (kOe) 1st
2nd 3th 4th
M (emu/g)
H (kOe)
Fig. 4. The multi-cycles hysteresis loops of BLFTO sample measured after pre-applied -10 kOe The spin pinning is the origin of the vertical hysteresis shift and exchange bias effect observed in sample. The magnitude of the exchange bias field can be calculated as HEB = - (Hc1 + Hc2)/2, where Hc1 and Hc2 are the negative and positive side fields, respectively.
The exchange bias field, which showed in Figure 3, decreases from 4.0 kOe to 3.2 kOe, for the first and second hysteresis loops, respectively. It worth noting that the strength of exchange bias field decreases with the increase of applied field. It also depends on the magnetic coupling between two phases. It is implied that the exchange bias effect is dependent on the phase ratio of two phases.
CONCLUSIONS
We have successfully synthesized a multiferroic Bi0.84La0.16Fe0.98Ti0.02O3 without any impurity phase. The crystal structure has been studied in detail using Rietveld refinement method revealed the multiphase rhombohedral and orthorhombic presence in
sample. The coexistence of multiphase crystal structure plays the important role on the magnetic properties of sample. The magnetic properties were improved with co-doped La and Ti to BiFeO3. The vertical hysteresis shift and the exchange bias effect were observed in sample. We proposed that the magnetic coupling at phase boundary is the origin of this effect. However, further investigation with another method is necessary to fully understand the contribution of phase boundary to the magnetic properties in this compound.
Acknowledgments
This work was supported by the ĐH2015 TN06-10 project of Thai Nguyen University.
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TÓM TẮT
CẤU TRÚC VÀ TÍNH CHẤT TỪ CỦA VẬT LIỆU GỐM ĐA TINH THỂ Bi0.84La0.16Fe0.98Ti0.02O3
Chu Thị Anh Xuân, Phạm Trường Thọ, Nguyễn Văn Đăng* Trường Đại học Khoa học – ĐH Thái Nguyên Vật liệu đa pha điện từ Bi0.84La0.16Fe0.98Ti0.02O3 được chế tạo bằng phương pháp phản ứng pha rắn.
Kết quả phân tích giản đồ nhiễu xạ tia X cho thấy vật liệu đồng tồn tại hai pha cấu trúc là R3c rhombohedral và Imma orthorhombic. Phương pháp phân tích cấu trúc Rietveld được sử dụng để xác định các tham số cấu trúc như hằng số mạng, góc liên kết Fe – O – Fe, và độ dài liên kết Fe – O. Kết quả đo đường cong từ trễ cho thấy vật liệu có tính chất sắt từ yếu ở nhiệt độ phòng.
Sự đồng tồn tại và cạnh tranh của hai pha cấu trúc ảnh hưởng mạnh lên tính chất từ của vật liệu.
Đặc biệt, chúng tôi quan sát thấy hiện tượng đường cong từ trễ bị dịch theo trục từ độ (hiệu ứng trao đổi hiệu dịch). Theo chúng tôi, hiệu ứng này có nguồn gốc bởi tương tác từ tại vùng biên pha cấu trúc.
Từ khóa: BiFeO3, Cấu trúc tinh thể, Tính chất từ, Vật liệu đa pha điện từ; đa pha cấu trúc.
Ngày nhận bài: 25/8/2017; Ngày phản biện: 13/9/2017; Ngày duyệt đăng: 30/9/2017
*Tel: 0983 009975; Email: dangnv@tnus.edu.vn
